1. Field of the Invention
The present invention relates to an angular-rate detecting apparatus for detecting an angular rate of a rotating body.
2. Description of the Related Art
A known angular-rate detecting apparatus is an angular rate sensor which detects a displacement of a mass portion due to a Coriolis force as an angular rate by utilizing the principle that a mass portion vibrating in a given direction undergoes a Coriolis force in accordance with an angular rate (for example, see Japanese Unexamined Patent Application Publication No. 2002-81939).
Such an angular rate sensor according to the known art includes three mass portions that are juxtaposed to one another on a substrate, and these mass portions are connected by support beams so as to be vibratable in the X-axis direction. Also, each support beam has fixing portions, each disposed between two of the mass portions so as to fix central longitudinal portions of the support beam to the substrate.
Also, the central mass portion includes a vibration generator disposed thereon, generating an electrostatic force between the mass portion and the substrate when an external drive signal (voltage) is applied to the generator. In this case, when the vibration generator vibrates the central mass portion with the electrostatic force in the X-axis direction, the outside mass portions adjacent to the central mass portion are vibrated in reverse phase. With this structure, according to the known art, the mass portions are kept in a constant resonant state while two of the mass portions adjacent to each other vibrate in reverse phase with the fixing portions functioning as nodes.
When a torque, for example, about the Y-axis is applied to the sensor while these mass portions are vibrating, each outside mass portion is displaced in the Z-axis direction since it undergoes a Coriolis force in accordance with its vibrating state. In this case, the outside mass portion has an angular rate detector disposed thereon in which the capacitance between the outside mass portion and the substrate varies in accordance with the displacement in the Z-axis direction. Thus, the angular rate detector detects the displacement of the outside mass portion as an angular rate about the Y-axis as a variance in capacitance.
In the meantime, according to the known art, the mass portions are kept in a constant resonant state while the central and outside mass portions are vibrated in opposite phases. However, each mass portion has not only a vibration mode in which two of the mass portions adjacent to each other vibrate in opposite phases (hereinafter, this vibration mode is referred to as the normal vibration mode) but sometimes also has a plurality of vibration modes. For example, when a large number of mass portions are connected, the number of vibration modes other than the normal vibration mode (that is, the number of vibration modes in which any two of mass portions adjacent to each other do not vibrate in opposite phases) increases.
Accordingly, in the known art, a vibration mode other than the normal vibration mode is likely to be excited, for example, at the time of starting up the sensor, as a result, a period of startup time from startup of the sensor to excitation of the normal vibration mode is longer or a drive signal at a high signal level (a high voltage) is needed to be inputted into the vibration generator in order to excite and maintain the normal vibration mode, thereby causing a problem of deterioration in performances at the time of startup or an increase in power consumption.
Also, when each mass portion vibrates continuously in a vibration mode other than the normal vibration mode, the amplitude and the vibration frequency of the vibration becomes unexpectedly unstable, thereby causing another problem of deterioration in detection accuracy or reliability required as a sensor.